THE OVUM AND
by a very transparent, perfectly structureless membrane, which represents a closed cell-membrane, having as little connexion with the ovary as with the layers of cells, and which is denominated vitellne membrane, It is as readily separated from the ovary as from the layer of cells, the latter, therefore, cannot be merely its epithelium.
If we now proceed to examine larger eggs from the ovary, such, for instance, as have attained a diameter of half an inch or more, and are already yellow-coloured, on their being divided across the centre under water, a white substance, the yelk-cavity, will be found in their interior. This cavity contains those cells, now in a higher stage of development, which in the first instance alone formed the contents of the egg. Around these a stratum of yellow substance, the proper yelk-substance, appears, and round this again lies the layer of cells. Globules may be recognised in the proper yelk-substance with the aid of the microscope, as in the same substance of the mature yelk. These globules, then, have been formed between the yelk-cavity and the layer of cells. The question, however, arises how this has been effected? The following may be supposed to be the mode of their production: — the innermost portion of the yelk, the yelk-cavity, is the part which is first formed, the innermost yelk-globules are therefore also the oldest, and the formation of the new yelk-globules takes place externally upon the internal surface of the layer of cells. If a small portion of the layer of cells be so placed under the microscope that the inner surface becomes turned towards the eye, and a spot be sought for at which a thin layer of yelk-substance is attached to it, it will be seen that the yelk-globules do actually become smaller in the proximity of the layer of cells, whilst in other respects they retain their general appearance. The smallest of them, which lie immediately upon the inner surface of the layer of cells, are even smaller than the cells of the layer itself. It is therefore extremely probable, that the formation of new yelk-globules takes place on the inner surface of the layer of cells, and that the globules then expand to their normal size somewhat quickly, for the stratum of small ones is but thin. Meanwhile new ones continue to form externally, until the yelk has reached its normal size. The formation of the canal leading from the yelk-cavity to the germinal vesicle may also be ex-
